Add '.vercel' to ignored content directories (#18855)

Just like `.next` is ignored, ignore `.vercel` as well.

Author: mischnic

crates/oxide/src/scanner/fixtures/ignored-content-dirs.txt

@@ -11,3 +11,4 @@ venv
 __pycache__
 .svelte-kit
 .pnpm-store
+.vercel